In Global Systems for Mobile Communication (GSM) wireless communication networks, subscribers frequently move between Location Area Codes (LAC's), one or many of which is served by a Visitor Location Register (VLR) of a network. As subscribers move between LAC's which are served by different VLR's, Inter-VLR location updates are performed to maintain the current location of the subscriber in the network. To perform Inter-VLR location updates as a subscriber enters a new LAC served by different VLR's, the subscriber is assigned a Temporary Mobile Subscriber Identity (TMSI) number so that exposure of the subscriber's International Mobile Subscriber Identity (IMSI) number is minimized. This helps to reduce exposure of the IMSI to possible fraudulent use.
During Inter-VLR location updates, TMSI's are re-assigned. GSM specifications recommend that the assignment of new TMSI's should be done using a ciphered channel in order to reduce the chance of interception. Therefore, in order to transmit a subscriber's new TMSI, the channel must first be ciphered using Base Station Subsystem Mobile Application Part (BSSMAP) Ciphering procedures. This requires that the network send a BSSMAP Cipher Mode Command message to a Base Station Subsystem (BSS), and then wait for a Cipher Mode Complete message before the TMSI can be delivered. Upon receipt of Cipher Mode Complete message, the network sends a new TMSI within a DTAP Location Update Accept message. The new TMSI is stored in a subscriber's mobile station which then confirms its receipt by sending a DTAP TMSI Reallocation Complete message.
Additionally, upon initial receipt of a Location Update Request message during Inter-VLR location updates, the TMSI stored by the mobile station will be unknown in the new VLR since the TMSI was allocated in a previous VLR. This requires that the network obtain the subscriber's IMSI from either the previous VLR or the mobile station.
As the number of cellular subscribers in a metropolitan area increases, the number of switching nodes in the area increases. This results in a higher rate of mobility between switching nodes and in the number of Inter-VLR location updates that must be performed. An increase in the rate of Inter-VLR location updates results in an increase in quantity of network work-time that must be expended handling subscriber mobility events such as IMSI retrieval, ciphering, and TMSI reallocation. This increase in the work required to handle mobility results in a decrease in the number of subscribers the network can support.
One method that has been proposed to reduce Inter-VLR location update work-time has been to use the concept of a "SuperCharger" to reduce messages sent between a VLR and an HLR. SuperCharger is described in a Patent Application entitled "Distributed Subscriber Data Manager in Wireless Networks from a Remote Perspective" filed on Aug. 30, 1996, and assigned Ser. No. 08/697,854, a continuation of same filed on May 29, 1997, and assigned Ser. No. 08/864,927, and in a Patent Application entitled "Distributed Subscriber Data Manager in Wireless Networks from a Central Perspective" filed on Aug. 30, 1996, and assigned Ser. No. 08/697,854, and a continuation of same filed on May 29, 1997, and assigned Ser. No. 08/864,926. The SuperCharger, however, does not reduce work-time that must be spent handling subscriber mobility events such as IMSI retrieval, ciphering, and TMSI reallocation messages sent between the network and the BSS, i.e., on an A-interface.
Accordingly, a continuing search has been directed to the development of methods for performing Inter-VLR location updates which do not require significant network work-time, particularly on the A-interface of a network and, therefore, which permit networks to support a greater number of subscribers than is possible using conventional methods.